In a large number of communications systems, and particularly wireless communications systems, the peak-to-average power ratio (PAP) plays a crucial role in determining the overall cost and performance of the infrastructure components, such as Base Transceiver Stations (“BTS”) for wireless systems. This is due to the fact that the BTS uses a power amplifier to amplify a signal prior to transmission. With a high PAP, the power amplifier used in the communications system must have sufficient performance headroom to amplify the entire signal without clipping any portion of the signal. Since the power amplifier is one of the most expensive components in a wireless communications network, it is desirable to reduce the PAP as much as possible without excessively distorting the signal so that lower performance (lower power and hence lower cost) power amplifiers may be used. The problem is further complicated in communications systems such as Code-Division Multiple Access (CDMA) where the PAP will actually increase as the number of users increase.
One solution to an excessively high PAP is to clip the signal peaks if they exceed a certain magnitude threshold, replacing the signal peak with a value equal to the magnitude threshold. This is known as “hard clipping” and is effective in reducing the PAP. Unfortunately, hard clipping introduces emissions lying outside of the frequency band of the signals. These emissions are commonly referred to as spurious out-of-band emissions. These spurious emissions can cause the communications system to fail adjacent and alternate channel emission specifications, which have strict signal power specifications. The spurious emissions may be reduced through the use of filters that only pass the frequency band (in-band) signals and block the out-of-band emissions. However, the use of such filters introduces a problem of their own: peak re-growth. Peak re-growth is the re-growth of some (or all) of the clipped signal peaks and is caused by the smoothing of the sharp transitions (which were caused by the hard clipping) by filtering.
In U.S. Pat. No. 6,236,864, a solution involving multiple iterations of hard clipping and filtering is presented to solve the problems of out-of-band spurious emissions and peak re-growth. The premise is that the peak re-growth does not result in signals with peaks as large as the original signal peaks, and, therefore, each time the signal is clipped and filtered, the output signal has a smaller PAP than the input signal. However, the multiple iterations can introduce an unacceptable amount of latency into the communications system if the number of iterations is large and the filters themselves can be expensive and hard to implement.
A proposed solution for high PAP in Orthogonal Frequency Division Multiplexing (OFDM) communications systems involves the use of Gaussian scaling windows to scale the magnitudes of the signal peaks exceeding a specified threshold. The Gaussian windows do not perform hard clipping, rather, the Gaussian windows perform a softer type of clipping that does not result in sharp transitions and consequently does not result in as much spurious out-of-band emissions. The use of Gaussian windows however, trades reduction of signal peaks for degraded bit-error rate (BER) performance and increased out-of-band emissions.
The need has therefore arisen for a solution in communications systems with a high PAP that does not introduce a significant amount of latency into the communications system, increase spurious out-of-band emissions, or significantly degrade system performance.